Polyesters, especially poly(ethylene terephthalate) (PET) are versatile polymers that enjoy wide applicability as fibers, films, and three-dimensional structures. A particularly important application for PET is for containers, especially for food and beverages. This application has seen enormous growth over the last 20 years, and continues to enjoy increasing popularity. Despite this growth, PET has some fundamental limitations that restrict its applicability. One such limitation is its tendency to generate acetaldehyde (AA) when it is melt processed. Because AA is a small molecule, AA generated during melt processing can migrate through the PET. When PET is processed into a container, AA will migrate over time to the interior of the container. Although AA is a naturally occurring flavorant in a number of beverages and food products, for many products, the taste imparted by AA is considered undesirable. For instance, AA will impart a fruity flavor to water, which detracts from the clean taste desired for this product.
PET is traditionally produced by the transesterification or esterification/polymerization of a terephthalate precursor (either dimethyl terephthalate or terephthalic acid) and ethylene glycol. If the end use application for the melt-polymerized PET is for food packaging, the PET is then subject to a second operation known as solid-state polymerization (SSP), whereby the molecular weight is increased and the AA generated during melt processing is removed. A widely used method to convert the SSP PET into containers consists of drying and remelting the PET, injection molding the polymer into a container precursor (preforms), and subsequently stretch blow-molding the preform into the final container shape. It is during the remelting of the PET to fashion the container preforms that AA is regenerated. Typical preform AA levels for PET processed in the most modern injection molding equipment is 6-8 .mu./g (ppm).
Historically, the impact of AA on product taste has been minimized by careful control of the melt processing conditions used to make containers or preforms, and by use of special processing conditions in polymer preparation. This approach is successful for most packages, where the taste threshold for AA is sufficiently high, or where the useful life of the container is sufficiently short. However, obtaining low AA carries with it a significant cost. That cost includes the need to carry out a separate processing step after the melt polymerization of PET (solid-state polymerization), the need for specially designed injection molding equipment, and the need to continually monitor the AA content during container production. For other applications, where the desired shelf-life of the container is longer, the product is more sensitive to off-taste from AA, or the prevailing environmental conditions are warmer, it is not possible to keep the AA level below the taste threshold by using these methods. For example, in water, the taste threshold is considered to be less than about 40 .mu./L (ppb), and often a shelf-life of up to two years is desired. For a PET bottle that can contain 600 ml of beverage, a preform AA content of 8 ppm can result in a beverage AA level greater than 40 ppb in as little as one month.
In addition to careful control of melt-processing conditions for PET, prior art methods include modifications to the injection molding process to minimize the thermal and shear heating of the PET; use of lower IV resins, and the use of lower melting PET resins. Each of these approaches have been only partially successful, and each suffer from their own limitations. For example, specially designed injection molding equipment entail higher capital cost for the equipment. Lower IV resins produce containers that are less resistant to environmental factors such as stress crack failure. Lower melting resins are achieved by increasing the copolymer content the PET resin. Increasing the copolymer content also increases the stretch ratio of the PET, which translates into decreased productivity in injection molding and blow molding.
Another prior art approach has been to incorporate additives into PET that will selectively react with, or scavenge, the AA that is generated. Thus, Igarashi (U.S. Pat. No. 4,837,115) discloses the use of amine-group terminated polyamides and amine-group containing small molecules. Igarashi teaches that the amine groups are effective because they can react with AA to form imines, where the amine nitrogen forms a double bond with the AA moiety. Igarashi teaches that essentially any amine is effective. Mills (U.S. Pat. Nos. 5,258,233; 5,650,469; and 5,340,884) and Long (U.S. Pat. No. 5,266,416) claim the use of various polyamides, especially low molecular weight polyamides. Turner and Nicely (WO 97/28218) claim the use of polyesteramides. These polyamides and polyesteramides are believed to react with AA in the same manner as described by Igarashi.
While these AA scavengers are effective at reducing the AA content of melt-processed PET, they suffer from their own drawbacks. In particular, relatively high loadings of the polyamides are needed to effect significant AA reductions, and a very significant yellowing of the PET occurs on incorporation of these amine-containing additives. This color formation is believed to be due to the color of the imine group itself, and is thus unavoidable. The yellow color formation inherently limits this approach to articles where the PET can be tinted to mask the color. Unfortunately, most PET articles in use today are clear and uncolored.
Therefore, there is a need for a simple and economical method for reducing AA content in polyester products without using special polyester, melt-processing equipment, or melt-processing conditions and without discoloring the polyester product.